1. Field of the Invention
The present invention relates generally to electron tubes, such as photomultiplier tubes, image intensifiers, and more particularly to an electron tube having a photocathode whose surface is deposited with alkali metal upon confining alkali metal vapor in the tube.
2. Description of the Related Art
Ceramics are generally used in a photomultiplier tube to electrically insulate a photocathode, dynodes, and an anode. Japanese Laid-Open Patent Publication No. SHO-62-150644 proposes coloring the ceramics, for example, black, to reduce the dark current of the photomultiplier tube.
The ceramics can be colored starting either with manganese (Mn) which is a reddish coloring dye, or with cobalt (Co) which is a bluish coloring dye. Cobalt is several times more expensive than manganese and also gives bluish tint to black-colored ceramics. Therefore, ceramics colored black with manganese are primarily used in LSI packages and vacuum tubes.
A ceramic is typically composed of Al.sub.2 O.sub.3, Si, Ti, Mn, Fe, Cr, and the like. Generally, Fe, Cr, Co, Mn, Ni, Cu, and the like are used to color the ceramic.
The surface of photocathode in a photomultiplier tube is formed by introducing an alkali metal vapor into an electron tube. The present inventors recognized that a great deal of alkali metal vapor were required to deposit the alkali metal on the surface of the photocathode. The inventors found that the need for a great deal of alkali metal vapor resulted from absorption of the metal vapor by colored ceramics which insulate and support the various electrodes. However, it is desirable to make this type of electron tube using only a minimal amount of alkali metal, because the lower the alkali metal content, the better the characteristics relating to photoelectric conversion sensitivity and dark current. Service life of the photomultiplier is also prolonged if the amount of alkali metal contained in the ceramics is reduced.